1. Field of the Invention
This invention is related to a solid state relay including a power MOSFET in which the circuitry of the relay floats when the power MOSFET is commanded OFF to prevent leakage currents from draining a battery. This invention is also related to the prevention of arcing when electrical connectors are unmated. This invention is also related to 42 volt electrical systems that can be used in automobiles or motor vehicles to reduce electrical losses.
2. Description of the Prior Art
Contacts carrying significant amounts of power will arc when disconnected. The amount of arc damage experienced by the contacts depends on their physical structure, the load current, the supply voltage, the speed of separation, the characteristics of the load (resistive, capacitive, inductive) as well as other factors.
Future automotive systems are expected to utilize 42 volts in order to reduce the load currents and the associated wiring losses. This increased voltage could cause significant arc damage to occur to the present connectors designed for 12-volt operation. To avoid the possible liabilities associated with catastrophic connector failure, automotive manufacturers are requesting a new connector design that can be hot-swapped some significant number of times. Twenty cycles is considered to be a minimum requirement.
To disconnect 42-volt power without significant damage requires interrupting about 1500-watts for many loads and as much as 15 KW for the main battery circuit. Present day modules used in automotive applications can consume more than 500 watts. Power supplies must deliver one or more kilowatts of energy. Conventional solutions require either that the current be shut off before the contacts are separated or unmated or employ a sacrificial contact portion. Cost, space, reliability, safety, performance and complexity of these conventional solutions make them unsuitable for applications, such as automotive electrical systems.
There are many things, known in the power utility profession, which will quickly extinguish an arc and many things known in the relay industry that will minimize arc damage to connectors and contacts. These can be found in literature such as Gaseous Conductors by James D. Cobine and the Ney Contact Manual by Kenneth E. Pitney. Most of these methods are not practical in typical smaller and separable electrical connectors such as those used in automobiles, computers and appliances. None of them will eliminate arcing. In fact, even contacts that are rated for current interruption use in such smaller power connectors will be destroyed by interrupting rated currents often enough or slowly enough. There is a finite life for existing connectors since arcing will occur and cause damage each time the connector is disconnected.
One approach that has been suggested is to include a relay in the electrical system that will be switched off prior to disconnection of an electrical connector. The relay could be incorporated in a junction box or other enclosure that must be opened before the connector can be disconnected. When the junction box or enclosure is opened, the relay would also be opened when this approach is employed. Such an approach would, however, require additional components for every electrical connector that might be unmated or mated under load, and as such would add complexity and cost to an automotive electrical system.
Another alternative that has been considered is to incorporate a switching component, such as a power MOSFET, in an electrical connector. Such a switching device would be switched off before arcing could occur. However, individual power MOSFET's do not possess the required combination of size, current carrying capacity and cost to make such a solution practical at the present time. In addition conventional power MOSFET's have not been widely accepted for use in automotive applications, because of leakage currents that can drain a battery when a large number of such devices are used in an automotive electrical system. U.S. Pat. No. 5,926,354 discloses a solid state relay and a circuit breaker that includes a power MOSFET. However, the solid state relay circuit disclosed therein includes a ground connection through which current can leak from the battery positive terminal to ground when the power MOSFET is commanded to the OFF state. It is believed that conventional solid state relays that employ a power MOSFET exhibit this leakage problem if those relays are used in a standard relay package with a standard pin configuration. A solid state relay in accordance with the instant invention eliminates the leakage problem for solid state relays in standard configurations.